Unit pixel of image sensor and light-receiving element thereof

ABSTRACT

Provided are a light-receiving element which has more capability of detecting wavelengths than that of existing silicon light-receiving elements and a unit pixel of an image sensor by using it. The light-receiving element includes: a light-receiving unit which is floated or connected to external voltage and absorbs light; an oxide film which is formed to come in contact with a side of the light-receiving unit; a source and a drain which stand off the light-receiving unit with the oxide film in between and face each other; a channel which is formed between the source and the drain and forms an electric current between the source and the drain; and a wavelength expanding layer which is formed in at least one among the light-receiving unit, the oxide film and the channel and forms a plurality of local energy levels by using strained silicon.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.15/885,757, filed Jan. 31, 2018.

1. Technical Field

The present disclosure relates to a unit pixel of an image sensor and alight-receiving element thereof, more particularly to a light-receivingelement with expanded light-absorbing wavelengths by splitting energylevels of silicon and a unit pixel of an image sensor using thelight-receiving element.

2. Description of the Related Art

An image sensor converts an optical signal into an image signal, which,when light is radiated onto a light-receiving unit inside a unit pixelof an image sensor chip, detects the light and its quantity incidentupon each of the unit pixels, thereby converting a photosignal togenerate an electrical signal and then transfers the electrical signalto an analog circuit unit and a digital circuit unit to form an image.

Existing image sensors can be categorized largely into charged coupleddevice (CCD)-type and complementary metal oxide semiconductor(CMOS)-type depending on their configuration and operational principles.The image sensor of CMOS-type is commonly called CMOS image sensor(CIS).

The image sensor of CCD-type transmits a signal electron group generatedby light in each of the pixels to an output unit by means of a pulseapplied to a gate, converts the group into voltage at the output unitand puts out the group in turn.

On the other hand, the image sensor of CMOS-type converts signalelectrons and holes generated by light in each of the pixels intovoltage inside the pixel while the voltage is connected to a signalprocessing unit such as decoders of each column and row and put out ofthe pixel by switching operation according to clock frequencies.

On the other hand, the image sensors can be categorized into activepixel sensors (APSs) and passive pixel sensors (PPSs) depending onwhether there exists an amplifier in the unit pixel.

Whereas the PPS, as a passive device which has no signal amplificationfunction in the pixel, outputs the current of the light-receivingelement directly towards its outside, thereby converting the currentinto voltage outside the pixel, the APS is an active pixel which has avoltage signal amplification function inside the pixel.

Since the PPS is generally composed of a single photodiode and a singleselection transistor, the PPS has advantageous effects of increasing theaperture ratio and growing the fill factor, which is related to thelight-receiving efficiency compared with the APS, which requires threeto five MOS transistors for an identical pixel size.

On the contrary, the PPS has photocurrent with low intensity andconverts the photosignal into the form of current, which is vulnerableto the external environment such as noise to use for signal processing,which accounts for its disadvantageous effect of generatingfixed-pattern noise (FPN).

Therefore, for an identical size of the pixel, the APS can obtain theimage signal which has less noise than that of the PPS because the APShas a plurality of the transistors in its unit pixel, although thelight-receiving unit of the APS gets smaller than that of the PPS interms of the size of the light-receiving unit.

Meanwhile, an electron-hole pair (EHP) is generated against a photonincident from the light-receiving unit of the unit pixel of the imagesensor while the generated electrons and holes are accumulated in thephotodiode, which is the light-receiving unit.

The maximum electrostatic capacity that can be accumulated in thephotodiode is proportional to its light-receiving area. In particular,it is physically limited to enlarge the light-receiving area of the CMOSimage sensor in that the CMOS image sensor has an area for arranging thetransistors accompanying the CMOS sensor larger than that of the CCDimage sensor. In the meantime, the photodiode used mainly as thelight-receiving unit of the image sensor has relatively smallelectrostatic capacity, and therefore, is easily saturated and noteasily subdivided in an analog way.

As a result, the unit pixel of the CMOS image sensor requires arelatively long period for accumulating photocharges so as to generatethe minimum charge for processing signals through the limitedlight-receiving area. Thus, the unit pixel having such a light-receivingunit is not suitable for the image sensor with a high-density/high-speedframe.

The band gap of silicon semiconductors is 1.12 eV and such siliconsemiconductors can detect light energy of which wavelength ranges from400 to 1,100 nm when used as the light-receiving element. In otherwords, such silicon semiconductors cannot detect light of whichwavelength is 400 nm or less or 1,100 nm or more and are not applied toany sensors for biometrics, motion recognition and the like, which makeuse of near infrared or infrared rays, despite their economicefficiency.

SUMMARY

To resolve the problems, the present disclosure provides thelight-receiving element and the unit pixel of the image sensor withexpanded light-absorbing wavelengths by forming strained silicon in anarea including the light-receiving unit which light enters, therebyefficiently splitting energy level of silicon.

To achieve the objectives, an embodiment of the present disclosureprovides the light-receiving element, including:

the light-receiving unit which is structured to be floated and absorbslight;

an oxide film which is formed to come in contact with a side of thelight-receiving unit;

a source and a drain which stand off the light-receiving unit with theoxide film in between and face each other;

a channel which is formed between the source and the drain and forms anelectric current between the source and the drain; and

a wavelength expanding layer which is formed in at least one among thelight-receiving unit, the oxide film and the channel and forms aplurality of local energy levels,

wherein the light-receiving unit is electrically insulated from thesource and the drain by means of the oxide film;

electrons excited when light is received flow into the light-receivingunit under the tunneling effect when at least one among the local energylevel conditions is met; and

the electric current along the channel is controlled according to changeof electric charge quantity in a gate or the light-receiving unit withthe inflow of the electrons.

Here, the light-receiving element can be formed on a silicon substratein which first-type impurities are doped.

Here, the source and the drain are formed on a well in which second-typeimpurities are doped, wherein the well can be floated.

Here, the wavelength expanding layer is formed including strainedsilicon and can split the energy levels of the conduction band and thevalence band according to the direction of the crystal lattice planes ofsilicon.

Here, the light-receiving element can include a plurality of thewavelength expanding layers, which are different from each other interms of at least one of their formation location and manufacturingprocess.

Here, the wavelength expanding layer can be formed by applying stressonto silicon and the interface of the oxide film.

Here, the wavelength expanding layer can be formed by bondingheteroatoms to or injecting ions into a silicon crystal which forms thelight-receiving unit or the channel.

Here, the size of the light-receiving unit can correspond to a deBroglie wavelength of an electron.

Here, the light-receiving unit can further include a terminal whichapplies gate voltage to the light-receiving unit so as to control thethreshold voltage and output signals by controlling the gap between theFermi level and the intrinsic level of the channel.

Here, the tunneling can occur in an oxide film zone between any one ofthe channel and the drain and the light-receiving unit.

An embodiment of the present disclosure provides the unit pixel of theimage sensor, including:

the light-receiving element which generates an electric current by usingchange of electric charge quantity caused by incident light; and

a reset element which removes residual electric charges from thelight-receiving element,

wherein the light-receiving element includes:

the light-receiving unit which is structured to be floated and absorbslight;

the source and the drain which stand off the light-receiving unit withthe oxide film in between;

the channel which is formed between the source and the drain and formsan electric current between the source and the drain; and

the wavelength expanding layer which forms a plurality of local energylevels,

and controls the electric current along the channel based on change ofelectric charge quantity in the light-receiving unit while electronsexcited when light is received flow into the light-receiving unit underthe tunneling effect when at least one among the local energy levelconditions is met.

Here, the reset element can remove residual charges from a diffusionwell where the light-receiving element is formed.

Here, the diffusion well can remain floated while the light-receivingelement operates.

Here, the reset element of the unit pixel of the image sensor isconnected to a diffusion layer in which the first-type impurities aredoped in high density on the diffusion well in which the first-typeimpurities are doped.

Here, the wavelength expanding layer is formed including strainedsilicon and can split the energy levels of the conduction band and thevalence band according to the direction of crystal lattice planes ofsilicon.

Here, the light-receiving element can include the light-receiving unitwhich corresponds to a de Broglie wavelength.

An embodiment of the present disclosure provides the unit pixel of theimage sensor, including:

the light-receiving element which generates an electric current by usingchange of electric charge quantity caused by incident light;

a selection element which outputs the electric current generated in thelight-receiving element to a unit pixel out; and

the reset element which removes residual electric charges from thelight-receiving element,

wherein the light-receiving element includes:

the light-receiving unit which is structured to be floated and absorbslight;

the source and the drain which stand off the light-receiving unit withthe oxide film in between;

the channel which is formed between the source and the drain and formsan electric current between the source and the drain; and

the wavelength expanding layer which forms a plurality of local energylevels, and controls the electric current along the channel based onchange of electric charge quantity in the light-receiving unit whileelectrons excited when light is received flow into the light-receivingunit under the tunneling effect when at least one among the local energylevel conditions is met.

Here, the selection element includes:

the source and the drain which are connected to the light-receivingelement and the unit pixel out, respectively; and

the gate to which selection signals are applied from the outside,

and can perform switching operation on the basis of the appliedselection signals.

Here, the wavelength expanding layer is formed including strainedsilicon and can split the energy levels of the conduction band and thevalence band according to the direction of crystal lattice planes ofsilicon.

Here, the light-receiving element can include the light-receiving unitwhich corresponds to a de Broglie wavelength.

According to the various embodiments of the present disclosure, the unitpixel of the image sensor can be manufactured which can detect lightenergy of which wavelengths are expanded compared with those ofconventional silicon-based image sensors.

Moreover, a separate manufacturing process is not necessary forphotodiodes for the unit pixel as in the general CCD- or CMOS-type imagesensors, thereby significantly reducing the size of the unit pixel ofthe image sensor. In addition, according to the present disclosure, itis possible to fabricate, by employing common MOSFET manufacturingprocesses, the light-receiving element which can output a considerableintensity of photocurrent from relatively small quantity of light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the tunnel junction light-receivingelement according to an embodiment of the present disclosure;

FIG. 2a illustrates the energy band of a common silicon single crystal;FIG. 2b illustrates the energy band of strained silicon; and the FIG. 2cillustrates the energy band of the common silicon crystal and thestrained silicon, comparing them;

FIG. 3 illustrates an example where the wavelength expanding layer isformed in the light-receiving unit and the channel according to anembodiment of the present disclosure;

FIG. 4 illustrates an example where energy levels are split when thesize of the light-receiving unit corresponds to the quantum mechanicalconfinement of electric charges according to an embodiment of thepresent disclosure;

FIG. 5 is a graph which plots wavelength characteristics of the tunneljunction light-receiving element according to an embodiment of thepresent disclosure;

FIGS. 6a through 6c are energy band diagrams which illustrate change ofoperation as light enters the tunnel junction light-receiving element100 according to an embodiment of the present disclosure;

FIG. 7 is a cross-sectional view of the unit pixel using the tunneljunction light-receiving element according to an embodiment of thepresent disclosure; and

FIG. 8 illustrates a circuit diagram of the unit pixel using the tunneljunction light-receiving element according to an embodiment of thepresent disclosure.

DETAILED DESCRIPTION

Embodiments according to the present disclosure will be described inmore detail hereinafter with reference to the accompanying drawings. Indescribing the present disclosure, any known technique will not bedescribed in detail when it is determined such description canunnecessarily make the subject matter of the present disclosure obscure.Any number used in describing the present disclosure such as a first, asecond and the like is nothing but an identifier for distinguishing oneelement from another one.

In addition, in the present Specification, when it is specified oneelement “is connected to” or “comes in contact with” another one,whereas the former element can be directly connected to or come incontact with the latter, it should be understood the two are possiblyconnected or come in contact with each other with still another elementin between as a medium unless otherwise contradictorily specified.

FIG. 1 is a cross-sectional view of the tunnel junction light-receivingelement according to an embodiment of the present disclosure.

In FIG. 1, the light-receiving element corresponds to the unit pixel andis achieved by using a tunnel junction element other than existingphotodiodes, wherein the tunnel junction element is a structure forwhich a thin insulating layer is bound between two conductors orsemiconductors, referring to an element which operates by the tunnelingeffect which occurs in the insulating layer. Just for information, thetunneling effect is one of quantum mechanical effects, meaning aphenomenon that a particle which moves under exertion of force havingpotential passes through an area of which potential energy is largerthan the kinetic energy of the particle itself.

The tunnel junction light-receiving element can be achieved according toan embodiment of the present disclosure in various structures. Although,as nonrestrictive examples, the element can be formed in the structureof n-MOSFET (metal-oxide-semiconductor field-effect transistor) orp-MOSFET, the element can use other structures of electronic elementssuch as JFET or JUGFET (junction gate field-effect transistor), HEMT(High-electron-mobility transistor) and the like, which can obtain thetunneling effect.

The tunnel junction light-receiving element 100 according to anembodiment of the present disclosure is achieved in the structure of aPMOS. The tunnel junction light-receiving element 100 includes an N-typewell 115 formed on a P-type substrate 110 and further includes the P+diffusion layer 120 which corresponding to the source of a common PMOSelectronic element and the P+ diffusion layer 130 which corresponds tothe drain thereof. The P+ diffusion layers 120, 130 are fabricated byinjecting high-density P-type impurities on the N-type well 115. The P+diffusion layers 120, 130 will be referred to as the source and thedrain, respectively, hereinafter.

Formed on the source 120 and the drain 130 are metallic contacts 121,131, which are connected to external nodes. The metallic contact 121 ofthe source 120 is connected to the outside via a metallic line 122.Likewise, the metallic contact 131 of the drain 130 is connected to theoutside via the metallic line 132. Unlike the common PMOS electronicelements, the N-type well 115 is formed in a floating structure and onlythe source 120 and the drain 130 are connected to the external nodes,which accounts for the difference between them in terms of structure.

In the silicon area between the source 120 and the drain 130 is formedthe channel 160 through which electric charges migrate while above thechannel 160 is formed the thin oxide film 140. The source 120 and thedrain 130 stand off the polysilicon gate 150 with the oxide film 140 inbetween and face each other. It is desirable to form 10 nm or less ofthe oxide film 140 in thickness for the sake of generating the tunnelingeffect, but not limited thereto.

On the oxide film 140 is formed the polysilicon gate 150, correspondingto the gate of the PMOS electronic elements, in which P- or N-typeimpurities are doped. Unlike the gate of the common PMOS electronicelements, the polysilicon gate 150 can have a floating structure. Inaddition, the upper portion of the polysilicon gate 150 is an area whichabsorbs light and any metallic silicide oxide layer is not formedtherein for the upper portion to operate. The polysilicon gate 150 willbe referred to as the “light-receiving unit” hereinafter.

Absorbed light has influence on the light-receiving unit 150 and theoxide film 140 and the channel 160 as well. Therefore, thelight-receiving unit 150, the oxide film 140 and the channel 160 arereferred to as a “light-absorbing layer”.

The N-type well 155 has a floating structure. Since the N-type well 115is floated, it is easier to detect change of electric charge quantitygenerated by incident light in the light-receiving unit 150.

The tunnel junction light-receiving element 100 can include alight-shielding layer 180. Being formed on the tunnel junctionlight-receiving element 100 save for the light-receiving unit 150, thelight-shielding layer 180 shields light from being absorbed in an areaother than the light-receiving unit 150, which is for photocharges inthe light-absorbing layer 190 to tunnel efficiently. The light-shieldinglayer 180 can be formed by a silicide and/or metal process while a maskcan prevent the light-shielding layer 180 from being formed on thelight-receiving unit 150 in its upper portion.

An electric field is formed between the source 120, the drain 130 andthe light-receiving unit 150 while the channel 160 is formed between thesource 120 and the drain 130 by light incident upon the light-receivingunit 150 when light is radiated onto the tunnel junction light-receivingelement 100. More specifically, EHPs are generated by light incidentupon the light-absorbing layer 190. In other words, when light havingenergy larger than the band gap or the energy to bind the dopedimpurities is radiated onto each of the light-absorbing layers 190,which means light of which energy is larger than the band gap or theenergy to bind the doped holes is radiated onto the light-receiving unit150 and the channel 160 with the oxide film 140, which shuts outmovement of charges in their equilibrium, as a boundary, a plurality ofEHPs are generated and the charges come into their free state.

At this point, the formed EHPs exist for a certain period as electronsand holes separately before they are recombined, which leads to a localincrease of the number of electrons, and therefore, of charge quantity.Such separated, or excited, electrons move freely outside the grainboundary of the light-receiving unit 150 and, as a result, the electronsare accumulated in proximity to the boundary of the light-receiving unit150 to come under the influence of the electric field. As the number ofthe connected electrons increases, the electric field gets intensified,which accelerates the electron accumulation in proximity to the boundaryof the light-receiving unit 150. As seen this phenomenon, when incidentlight is more intense, more EHPs are generated and a more intenseelectric field is generated.

After that, the excited electrons flow from the drain 130 and thechannel 160 and the like into the light-receiving unit 150 by thetunneling effect of the oxide film 140 when the energy level conditionsare met. Therefore, the light-receiving unit 150 changes in terms of itstotal charge quantity. In other words, because the charge quantity ofthe channel 160 increases the same as the number of electrons flowing inby the tunneling effect, by the effect of threshold voltage modulationaccording to the change of the charge quantity of the light-receivingunit 150 by means of the electron inflow, conductance of the channel 160changes between the source 120 and the drain 130 and a currentcorresponding to the changed conductance flows.

At this point, according to an embodiment of the present disclosure, thewavelength expanding layer 170 is formed, which expands the wavelengthsthe tunnel junction light-receiving element 100 absorbs by forming aplurality of local energy levels in the light-absorbing layer 190including the light-receiving unit 150. The wavelength expanding layer170 includes the strained silicon layer caused by applying stress to orinjecting ions into the light-receiving unit 150, the interface of theoxide film 140 or the interface of the substrate (or the channel 160)and, unlike conventional silicon, can detect light of wavelengths commonsilicon photodiodes cannot detect because it generates a plurality ofthe local energy levels by splitting each energy level of the valenceband and the conduction band.

FIG. 2a illustrates the energy band of a common silicon single crystal;FIG. 2b illustrates the energy band of strained silicon; and the FIG. 2cillustrates the energy band of the common silicon crystal and thestrained silicon, comparing them.

For information, the crystal direction of a silicon single crystal isdetermined as (1), (2), (3) and the like according to its latticearrangement. As a result, in its energy band, the lowest level of theconduction band (CB) does not correspond to the highest level of thevalence band (VB) in the k-space. Therefore, the electron excited bylight is a material in the indirect transition mode which should employa phonon as a medium. Single crystal silicon shows infinitesimalspin-orbit interaction, and, therefore, the split-off bands (spinsplitting bands) to heavy holes (HH) and light holes (LH) bands aresimilar to each other in the sense of energy (Δ=44 meV). Thus, asillustrated in FIG. 2a , the band gap energies of common single crystalsilicon have an identical value (Eg=1.12 eV). In strained silicon,however, the energy level is split to HHs and LHs of the CB.Accordingly, the CB shows, as illustrated in FIG. 2b , the local energylevels generated based on the crystal directions, (1), (2) and (3).Since the band gap energies, Eg1, Eg2, Eg3 and Eg4, according to thesplit energy levels are different from the existing band gap energy, Eg,the wavelength of the light energy detected in the strained siliconlayer expands the wavelength of the light energy detected in the commonsilicon layer. In short, in FIG. 2b , four light energies can beabsorbed.

Meanwhile, it is assumed the strained silicon is formed by applyinguniaxial tensile stress to a single crystal silicon. Besides, thewavelength expanding layer 170 in which the strained silicon is formedcan be fabricated according to many techniques. As a nonrestrictiveexample, the wavelength expanding layer 170 can be fabricated byapplying stress to the surface of the oxide film, polysilicon and/orsubstrate (or the channel area), binding heteroatoms (for example,germanium, Ge) or injecting ions (for example, phosphorus, P, boron, B,nitrogen, N, gallium, Ga and the like). Furthermore, the strainedsilicon can be formed by controlling stress for heat treatment byincreasing the dopant density of the oxide film, polysilicon and/orsubstrate.

In particular, when the size of the light-absorbing layer 190 decreases,the energy levels per area of the local energy levels formed in thewavelength expanding layer 170 get involved more aggressively in theprocess of electron excitation. In a PN junction diode as thelight-receiving element of the common image sensor, it is advantageousto enlarge the area onto which light is radiated because the area of thelight-absorbing layer has direct influence on its light-receivingefficiency. On the contrary, in the tunnel junction light-receivingelement 100 of the present disclosure, the light-absorbing layer with alesser area can detect light energy with wider wavelengths, whichaccounts for an advantageous effect of the present disclosure.

Two of the wavelength expanding layers 170 are illustrated in FIG. 1.However, FIG. 1 just shows an example of the present disclosure. Thetunnel junction light-receiving element 100 can control thelight-absorbing peak wavelength by changing local energy levels andsplit states by modifying the number of the wavelength expanding layer170, its location, its fabricating process and the like. For example, itis possible to detect light energy with more wavelengths by increasingthe number of the states of local energy levels by forming a pluralityof the wavelength expanding layers and modifying the fabricating processof each of the wavelength expanding layers. It can be experimentallydetermined in the fabricating process according to the variouscharacteristics of the light-receiving element.

FIG. 3 illustrates an example where the wavelength expanding layer isformed in the light-receiving unit and the channel according to anembodiment of the present disclosure. Referring to FIG. 3, the electronsexcited in the wavelength expanding layer 171, 172 when light enters thewavelength expanding layer 171, 172 migrate to at least one among theoxide film 140, the interface of the channel 160 and the drain 130 andflow into the light-receiving unit 150 when at least one among aplurality of the local energy level conditions is met. At this point,because driving voltage is applied to the drain 130, the electronscontinuously flow into the light-receiving unit 150 after tunnelingthrough the oxide film 140. Therefore, the tunnel junctionlight-receiving element 100 does not require an independent amplifyingelement to amplify signals.

Meanwhile, the channel 160 is designed during the fabricating process ofthe tunnel junction light-receiving element 100 in order for the channel160 to be a state immediately before pinch-off. The channel 160 isformed due to the voltage difference between the source 120 and thedrain 130 while a depletion layer is formed around the source 120, thedrain 130 and the channel 160. In order to be immediately beforepinch-off without applying external voltage to the source 120 and thedrain 130, the channel 160 is fabricated by controlling the W/L ratio,the ratio of its width to its length. The W/L ratio can be, as anonrestrictive example, ½.

In addition, the size of the light-receiving unit 150 can be used forcontrolling the wavelengths of absorbed light as the light-receivinglayer which includes the strained silicon does. As described above, asthe light-receiving unit (and/or the light-absorbing layer) has a lesserarea, the size of the electric field increases between the source 120,the drain 130 and the light-receiving unit 150. This leads to anincrease of the probability for photoexcited electrons to tunnel throughthe oxide film then from resultant photocurrent, which increases thephotoelectric conversion efficiency. In particular, when the size of thelight-receiving unit 150 is designed so that the size corresponds toquantum mechanical confinement of electrons, the energy of electrons andholes inside the light-receiving unit becomes no longer continuous as inthree-dimensional space, by the quantization effect, but quantized sothat their energy levels are split with certain intervals in between.

FIG. 4 illustrates an example according to an embodiment of the presentdisclosure where energy levels are split when the size of thelight-receiving unit is affected by quantum mechanical confinement ofelectric charges. Referring to FIG. 4, the energy levels of silicon aresplit by the strained silicon in the wavelength expanding layer whileeach of the split energy levels is further split at an interval of acertain value of energy by quantization caused by the quantum mechanicalsize effect. Accordingly, effective signals can be detected from lightwith more wavelengths.

Here, in the description above, that the light-receiving unit isfabricated so that it corresponds to quantum mechanical confinement ofelectric charges includes a case where the size of the light-receivingunit is identical to the size of their de Broglie wavelength (about 10nm) and any fabrication within the range similar thereto as well.

FIG. 5 is a graph which plots wavelength detection characteristics ofthe tunnel junction light-receiving element 100 according to anembodiment of the present disclosure. The image sensor of common siliconmaterials detects effective signals from 400 to 1,100 nm. On thecontrary, the tunnel junction light-receiving element 100 according toan embodiment of the present disclosure has, as illustrated in FIG. 5,peak characteristics of 900 nm or more, based on a plurality of bandgaps according to a plurality of local energy levels, and can detecteffective signals at 1,200 nm or more.

FIGS. 6a through 6c are energy band diagrams which illustrate change ofoperation as light enters the tunnel junction light-receiving element100 according to an embodiment of the present disclosure. In FIGS. 6athrough 6c , it is assumed the wavelength expanding layer is formed inthe channel (or the interface of the N-type well).

When light enters EHPs are generated in the light-receiving layer 190 asillustrated in FIGS. 6a and 6b , wherein a plurality of local energylevels are generated in the channel where the wavelength expanding layer170 is formed. When power supply voltage, VDD, is applied, such excitedelectrons tunnel through, driven by the electric field, the oxide film140 in the channel 160, the drain 130 and the like to make the interfacebetween the light-receiving unit 150 and the oxide film 140 depleted.When the interface is thus depleted by tunneling of electrons, thechannel 160 is formed under the effect of threshold modulation accordingto change of charge quantity in the light-receiving unit 150.

After that, when light intensity is diminished or shut out, asillustrated in FIG. 6c , the electrons which have completed tunnelingtunnel through the source 120 and the channel 160 again and chargequantity in the light-receiving unit 150 is restored to its originalstate. Accordingly, the formed depletion layer decreases and, at thesame time, the photocurrent formed in the channel decreases.

However, after light is shut out, the excited electrons may not beremoved altogether and can reside in the diffusion well (or thesubstrate of the N-type well 115 and the like) to make operationalerrors such as delayed signals. Therefore, the tunnel junctionlight-receiving element according to an embodiment of the presentdisclosure can further include the reset element to remove the residualelectrons altogether.

FIG. 7 is a cross-sectional view of the unit pixel 200 using the tunneljunction light-receiving element 100 according to an embodiment of thepresent disclosure. In FIG. 7, the unit pixel 200 includes the tunneljunction light-receiving element 100 and the reset element 300 describedwith reference to FIGS. 1 through 4.

Here, the reset element 300 can be composed of various elements andformed by using existing MOSFET structures. In such cases, the N+diffusion layer 210 is formed by injecting high-density N-typeimpurities onto the N-type well 115 of the tunnel junctionlight-receiving element 100 while the reset element 300 can be achievedthrough the existing MOSFET fabricating process.

According to an embodiment of the present disclosure, the reset element300 can be achieved into an NMOS, wherein the source 320 of the resetelement 300 is connected to the N+ diffusion layer 210 on the N-typewell 115 while bias voltage(NW_BIAS) is applied to the drain 330 of thereset element 300. Here, the bias voltage of the reset element 300 canbe used to remove the residual electrons in the N-type well 115 bytoggling it, or switching it on and off, but is not limited thereto. Asa nonrestrictive example, the VDD of the tunnel junction light-receivingelement 100 is about 2.7 V, wherein the NW_BIAS of the reset element 300is about from 2.2 to 2.7 V. When the NW_BIAS is applied to the drain 330of the reset element 300, a certain amount of voltage is applied to theN-type well 115 of the tunnel junction light-receiving element 100connected to the source 320 of the reset element 600 and the residualcharges in the N-type well 115 are removed, initializing the tunneljunction light-receiving element 100.

According to an embodiment of the present disclosure, in the tunneljunction light-receiving element 100, signal characteristics can becontrolled including sensor sensitivity, dynamic range, output signallinearity and the like by intentionally controlling gate voltage, VGG,through a terminal connected to the light-receiving unit 150. Forexample, when 0 V of the gate voltage is applied, a hole accumulationregime is formed in the silicon interface in the sense of potentialbecause its Fermi level and intrinsic level are overlapped each otherwhile a current flows constantly in the along the channel. In thisstate, the current flowing in the channel is composed mostly of a darkcurrent. Therefore, light-receiving efficiency can be easily enhanced byapplying 0 V or more of the gate voltage to the light-receiving unit 250to decrease the dark current component and increase the signal currentcomponent generated by reaction with light.

FIG. 8 illustrates a circuit diagram of the unit pixel 400 using thetunnel junction light-receiving element 100 according to an embodimentof the present disclosure. The unit pixel 400 according to an embodimentof the present disclosure, as illustrated in FIG. 8, includes the tunneljunction light-receiving element 100, the reset element 300 and theselection element 500.

Like the reset element 300, the selection element 500 can be composed ofvarious elements and formed by using the existing MOSFET structure.Therefore, fabrication costs can be reduced and fabrication processescan be simplified in that the tunnel junction light-receiving element100, the reset element 300 and the selection element 500 included in asingle piece of the unit pixel can be achieved through a single processof MOSFET fabrication.

The source 120 of the tunnel junction light-receiving element 100 isconnected to the power supply voltage, VDD while the source 120 comes incontact with the source 320 of the selection element 500. Although thesource 120 and the drain 130 of the tunnel junction light-receivingelement 100 have a symmetrical structure and are identical to eachother, the area which is connected to VDD or an external charge sourceis designated as source in this Specification and the scope of claims ofthe present disclosure. However, a person skilled in the art may easilyunderstand the scope of rights of the present disclosure shall range upto the opposite case.

The source 520 of the selection element 500 is connected to the drain130 of the tunnel junction light-receiving element 100 and the drain 530is connected to the unit pixel out to switch on and off the output ofthe tunnel junction light-receiving element 100. Control signals (SEL)to switch on and off the selection element 500 are applied through thegate 550.

When the selection element 500 is in the unselected state or it isswitched off, the unit pixel 400 is reset by the reset element 300. Whenthe reset element 300 is completely initialized, the tunnel junctionlight-receiving element 100 is operated by operating the selectionelement 500. Because all the residual charges in the floated N-type well115 are removed prior to operating the tunnel junction light-receivingelement 100, dark current generation due to the residual charges can beprevented.

To the N-type well 115 is connected the source of the reset element 300.However, when the tunnel junction light-receiving element 100 isoperating under the condition that reset is completed, in other words,when the selection element 500 is switched on, a photocurrent is notlost because this is equivalent to the condition that the N-type well isfloated.

As described above, the tunnel junction light-receiving element 100according to an embodiment of the present disclosure can generatehundreds to thousands of times of a photocurrent as compared withexisting photodiodes from the same amount of light. More specifically, aphotodiode distinguishes signal intensity only by the charge quantityaccumulated in its electrostatic capacity. On the contrary, the tunneljunction light-receiving element 100 can control the current in thechannel because change in the charge quantity by light in thelight-receiving unit 150 causes an electric field. Moreover, signals areamplified in the light-receiving element itself because necessarycharges are infinitely supplied through the channel and the drain.Therefore, the image sensor can be miniaturized without the necessity ofdesigning an independent signal amplifying element.

Furthermore, the tunnel junction light-receiving element 100 accordingto an embodiment of the present disclosure can be applied in biometrics,motion recognition, three-dimensional object recognition and the likebecause it generates a plurality of local energy levels, therebyexpanding the wavelengths from which effective signals can be detected.Meanwhile, although it is described above the tunnel junctionlight-receiving element 100 is achieved into the PMOS structure, it isobvious the element can have the NMOS structure. In addition, the tunneljunction light-receiving element 100 can have the light doped drain(LDD) structure. The light-receiving element 100 fabricated by the LDDprocess can reduce hot carrier generation due to the short channeleffect.

Description thus far is nothing but exemplary and it should beunderstood a person skilled in the art can modify and change the presentdisclosure within the scope thereof. Embodiments in the presentSpecification are used just for describing the technical thoughts of thepresent disclosure and not for limiting them. The scope of the technicalthoughts of the present disclosure should not be limited by suchembodiments. Therefore, the scope of rights of the present disclosureshould be construed by the scope of the Claims and all technicalthoughts within the scope should be construed as included in the scopeof rights of the present disclosure.

What is claimed is:
 1. A unit pixel of an image sensor, comprising: alight-receiving element which generates an electric current by usingchange of electric charge quantity caused by incident light; and a resetelement which removes residual electric charges from the light-receivingelement, wherein the light-receiving element comprises: alight-receiving unit which is structured to be floated and absorbslight; a source and a drain which stand off the light-receiving unitwith an oxide film in-between; a channel region formed between thesource and the drain, and an electric current formed between the sourceand the drain; and a wavelength expanding layer which forms a pluralityof local energy levels, and controls the electric current along thechannel region based on change of electric charge quantity in thelight-receiving unit, while electrons excited when light is receivedflow into the light-receiving unit under a tunneling effect when a localenergy level condition of at least one of the plurality of local energylevels is met, and wherein the wavelength expending layer split theenergy levels of the conduction band and the valence band according tothe direction of a crystal lattice arrangement of the light-receivingunit.
 2. The unit pixel of the image sensor of claim 1, wherein thereset element removes residual charges from a diffusion well where thelight-receiving element is formed.
 3. The unit pixel of the image sensorof claim 2, wherein the diffusion well remains floated while thelight-receiving element operates.
 4. The unit pixel of the image sensorof claim 2, wherein the reset element of the unit pixel of an imagesensor is connected to a diffusion layer in which a first-type ofimpurities are doped in high density on the diffusion well in which thefirst-type of impurities are doped.
 5. The unit pixel of the imagesensor of claim 1, wherein the wavelength expanding layer comprisesstrained silicon.
 6. The unit pixel of the image sensor of claim 1,wherein the light-receiving element comprises the light-receiving unitwhich corresponds to a de Broglie wavelength.
 7. A unit pixel of animage sensor, comprising: a light-receiving element which generates anelectric current by using change of electric charge quantity caused byincident light; a selection element which outputs the electric currentgenerated in the light-receiving element to a unit pixel out; and areset element which removes residual electric charges from thelight-receiving element, wherein the light-receiving element comprises:a light-receiving unit which is structured to be floated and absorbslight; a source and a drain which stand off the light-receiving unitwith a oxide film in between; a channel region which is formed betweenthe source and the drain, and an electric current formed between thesource and the drain; and a wavelength expanding layer which forms aplurality of local energy levels, and controls the electric currentalong the channel region based on change of electric charge quantity inthe light-receiving unit while electrons excited when light is receivedflow into the light-receiving unit under a tunneling effect when a localenergy level condition of at least one of the plurality of local energylevels is met, and wherein the wavelength expanding layer splits theenergy levels of the conduction band and the valence band according tothe direction of a crystal lattice arrangement of the light-receivingunit.
 8. The unit pixel of the image sensor of claim 7, wherein theselection element comprises: a source and a drain which are connected tothe light-receiving element and the unit pixel out, respectively; and agate to which selection signals are applied from the outside, andperforms switching operation on the basis of the applied selectionsignals.
 9. The unit pixel of the image sensor of claim 7, wherein thewavelength expanding layer comprises strained silicon.
 10. The unitpixel of the image sensor of claim 7, wherein the light-receivingelement comprises the light-receiving unit which corresponds to a deBroglie wavelength.